The development of small, efficient optical transmission lines such as optical fibers, has lead to widespread use of optical communication in many applications requiring, e.g., long distance, high data rate communication such as telecommunications. Optical fibers typically include a transparent core surrounded by a transparent cladding material having a refractive index lower than that of the core. Further, optical fibers (core and cladding) are typically coated with a polymer buffer layer, which is 250 microns in diameter, and an outer polymer jacket to further protect the optical fibers and provide mechanical strength. Fiber optic transmission lines provide low cost, compact, low EMI (electromagnetic interference), and high-speed data transmission over significant distances.
Typically, an optoelectronic transceiver package comprises a receiver and/or a transmitter interfaced with a connector to optical fibers. In general, the transmitter includes a semiconductor die with light sources that emit light signals which are transmitted through optical fibers. A variety of light emitting diodes (LEDs) and lasers may be used as light sources. For instance, a vertical cavity surface emitting laser (VCSEL) is a specialized laser diode that has been developed to provide improved efficiency and increased data speed in fiber optic communication. By design, a VCSEL emits its coherent energy perpendicular to the boundaries between semiconductor layers. A VCSEL typically has an emitting area about 10-15 microns in diameter, and light is coupled into an optical fiber (typically having about a 50 micron diameter core for multimode fiber).
Further, the receiver comprises a semiconductor die with light detectors (e.g., photodiodes) that receive light signals from optical fibers. Generally, the allowable photodiode diameter depends on the speed of the signal. For a 10 Gb/s signal, the photodiode is typically about 35 microns (or less) in diameter. As the signal speed increases, the photodiode diameter must be decreased to reduce the capacitance of the detector.
When optical transceivers (and other optical devices) are mounted on a PCB (printed circuit board), it is generally desirable to position the optical fibers parallel to the surface of the PCB (as opposed to perpendicular to the PCB). In this manner, a plurality of PCBs (having optical fibers and devices mounted thereon) can be closely spaced in parallel to each other. For instance, optoelectronic devices (e.g., VCSEL array and PD (photo detector) array) can be mounted on edge, parallel to each other, using a silicon nitride sub mount or on a flexible circuit.
When an optical fiber is disposed parallel to the surface of the PCB, there are various coupling techniques that may be employed for coupling light between the light sources/detectors and optical fibers. For example, in optical devices where the semiconductor light sources are top or bottom surface emitters (e.g., VCSEL), one coupling technique is to position an optical fiber parallel to a PCB and provide a 90 degree bend so that the end of the fiber can be butted to the light source or detector. This method requires a large spacing between PCBs because of the large minimum bending radius of the optical fibers and results in increased light loss, which may not be acceptable for various applications.
Other coupling techniques include “side-coupling” methods wherein an end portion of optical fiber is disposed adjacent to the light source/detector and light (which is emitted perpendicular to the axis of the fiber) is coupled into and out of the optical fiber with, or without, the use of mirrors. For example, FIGS. 1a and 1b illustrate a conventional side-coupling method for coupling light to and from an optical fiber from the side thereof by providing an acute angular cut along the end of the optical fiber, such as disclosed, for example, in U.S. Pat. No. 4,092,061, which issued to D. Stigliani on May 30, 1978, entitled Side-Coupling of Light For An Optical Fiber, which is incorporated herein by reference.
More specifically, as shown in FIGS. 1a and 1b, an optical fiber 1, which comprises a fiber core 2 surrounded by a transparent cladding material 3, comprises a reflective acute angular facet 4 formed on an end thereof, which serves as a mirror for side-coupling light to/from an optoelectronic device 5 (e.g., a top or bottom surface emitter light source, detector). The optical fiber 1 is brought in parallel to the surface of an optoelectronic device 5 (or parallel to a module, chip, etc., comprising the device 5), the surface being substantially parallel to fiber axis 6, such that the optoelectronic device 5 is aligned adjacent the side of the optical fiber 1 opposite an inner facing surface of the reflective facet 4. A reflective material is deposited on an outer surface of the facet 4.
With the side-coupling method depicted in FIGS. 1a and 1b, the light emitted in a plane perpendicular to the fiber central axis 6 is preferably reflected into the optical fiber core 2 substantially parallel to the fiber central axis 6. Further, the light traveling within the fiber parallel to the fiber axis 6 toward the reflective angular cut 4 is reflected out of the fiber core 2 through the cladding layer 3 to a detector. As illustrated in FIG. 1a, the curved fiber optic cladding material 3, which is disposed between the optoelectronic device 5 and the inner surface of the cut end 4 of the fiber core 2, acts as a cylindrical lens to partially collimate the light from a light source into the fiber core 2 as well as reduce the divergence of the light propagating from the fiber toward the detector 5.
The optoelectronic device 5 may be positioned face (or junction) up (as shown in FIGS. 1a and 1b) or face (or junction) down if the substrate is removed or adequately transparent for the wavelength of light of interest. For high speed electrical signals, there are a number of potential advantages in mounting the optoelectronic device face down and using solder bumps to attach the optoelectronic device directly to a VCSEL driver or photodiode amplifier chip, although subsequent assembly and alignment is easier with the optoelectronic device mounted face up.
Various methods for coupling light to and from an optical fiber using a 45 degree beveled facet on the end of the fiber are described in U.S. Pat. Nos. 4,329,659, 5,163,113, 6,031,953, 6,081,637, and 6,389,202.
The light-coupling systems and methods described in the above patents all suffer from the disadvantages described below. One disadvantage is that the closest possible spacing between the optoelectronic device 5 and the center of the optical fiber 1 is limited by the radius of the optical fiber 1, including both the core 2 and cladding layer 3.
Another disadvantage is that unless an index matching material is used, light being coupled out from the fiber 1 to a photodetector is spread-out more in the direction parallel to the fiber axis 6 than perpendicular to the fiber. This results in asymmetric divergence which makes it difficult to use further optics to focus the light. Even if an index matching material is used between the optical fiber (having a 45 degree facet at an end thereof) and the photodiode, the divergence of the light due to the optical travel distance will cause the spot size to increase.
For example, the full width half maximum distribution of a VCSEL is about +/−15 degrees in air. With a standard optical fiber having a 50 micron core diameter and standard cladding layer, the total diameter of the optical fiber is 125 microns. Thus, assuming the outer surface of the optical fiber directly contacts the photodetector surface, the optical path from the center of the fiber core to the photodetector would be about 62.5 microns long. If the optical index of refraction is 1.5, the spot size would increase by 11 microns per side, or a 50 micron diameter spot would increase to 72 microns in diameter due to the propagation of the unguided light for 62.5 microns. This large spot size will not be acceptable for future high speed optical communications systems where lower optical coupling losses are needed.
In the future, multimode optical fibers with smaller core diameters, such as 30 microns, may be used. In such case, the increase in the spot diameter by the propagation of unguided light through the cladding layer will contribute an even larger proportion to the final spot size.
Therefore, a means is needed for minimizing the distance between the core of the fiber and the optoelectronic device when side-coupling light to maximize the coupling of light to and from the fiber or fiber array to the optoelectronic device or optoelectronic device array.
Typically, when building integrated optical devices, optical fibers are mounted and secured into V-groove channels that are etched in a silicon substrate. For instance, FIG. 3 is an exemplary end-view of a conventional silicon V-groove array, which may be employed for mounting an array of optical fibers. The silicon V-groove array comprises a silicon substrate 20 having a plurality of V-groove channels 21 formed on a side thereof. An optical fiber 22 (comprising a core 23 and cladding 24) is secured in each of the V-groove channels 21 using known methods. This mounting method enables the central axes of the optical fibers in the array to be precisely spaced on a desired center C, which coincides with the distance between the points of convergence of the side walls of each V-groove 21.
The above fiber spacing/mounting technique can be used in various applications. For instance, such method may be used to precisely align the beveled end of an optical fiber to a laser diode for side-coupling light, such as disclosed in U.S. Pat. No. 5,163,113, issued to P. Melman on Nov. 10, 1992, entitled Laser-To-Fiber Coupling Apparatus.
Further, V-groove channel mounting methods may be used to form optical connectors. For example, FIG. 2 is a perspective view of a conventional optical fiber array connector 10. The connector 10 comprises two plates 11 and 12 (e.g., silicon plates) each having an array of optical fiber support channels 11a, 12a (V-grooves) formed on a surface thereof, corresponding to a longitudinal direction of optical fibers to be mounted therein. A plurality of optical fibers 13 are secured in corresponding channels 11a, 12a, between the plates 11, 12 using known clamping and bonding methods.
In general, a connector such as shown in FIG. 2 based on a silicon v-groove array is formed by: (1) etching V-groove channels into a silicon substrate and dicing silicon plates (having the channels) out from the wafer; (2) bonding the optical fiber(s) between corresponding V-grooves of top and bottom plates; and then (3) grinding and polishing the mating end of the connector so that the ends of the optical fiber(s) are coplanar with the edges of the v-groove plates 11, 12. For a connector that will not be permanently joined with an index matching material, it is desirable to have the optical fibers project slightly beyond the edges of the v-groove plates to ensure that there is no gap between the connected optical fibers.
Silicon V-channel arrays are preferably employed for forming silicon spacing chips and connectors such as shown in FIGS. 2 and 3 because the silicon v-groove arrays can be readily fabricated with high precision via anisotropic etching of single crystalline Silicon. More specifically, the formation of V-grooves in silicon is based on knowledge that the crystal of the silicon wafer has different atomic densities per unit area on different surfaces (100, 110, 111) of the crystal lattice, and that the etching rates vary along the different directions of the crystal lattice. Further, silicon is a very rigid material with a low thermal coefficient of expansion, which properties render silicon ideal for mounting optical fibers.
Methods for forming V-groove channels in silicon substrates are well known and are disclosed, for example, in Fiber-Optic Array Splicing with Etched Silicon Chips, by C. M. Miller, The Bell System Technical Journal, Vol. 57 No. 1, January 1978, pp. 75-90, Accurate Silicon Spacer Chips for an Optical Fiber Cable Connector, by C. M. Schroeder, The Bell System Technical Journal, Vol. 57, No. 1, January 1978, pp. 91-97, and as disclosed in Wet Bulk Micromachining, Chapter 4, Fundamentals of Microfabrication, by Marc Madou, CRC Press, 1997, all of which are incorporated herein by reference.
For connectors that can be attached/detached to/from other connectors and devices, alignment pins or other structures are preferably formed to enable precise alignment. For permanent connections, an optically transparent adhesive, such as a UV (ultraviolet) cured glue, can be applied to the end of the optical fiber(s) and the edge of the v-groove substrate, wherein the assembly is “actively aligned”, i.e. the light transmission is monitored while adjusting the relative positions of one assembly to a second assembly to optimize the coupling efficiency. When the alignment is satisfactory, if a UV glue is used, the join is exposed to UV light to cure the join.
The use of UV glue has the significant advantage that the joint can be made rapidly in the alignment jig with no temperature excursion. It is preferably to make the UV glue layer as thin as possible because the light coupling efficiency through the joint will decrease as the adhesive thickness increases due to absorption by the glue and divergence of light which is no longer confined to the optical fiber. This leads to a significant problem because if an array of fibers held by silicon V-groove substrates is being joined to another assembly which does not transmit UV light, it is difficult or impossible to properly expose and cure the UV glue forming the join if the layer is thin, such as less than 5-10 microns. Additionally, for a connector which uses alignment pins, it can be difficult to see the cavity into which the guide pins go from above when bring the v-groove blocks together.
The present invention provides a solution to all the above-mentioned problems, and others, associated with conventional side-coupling techniques and conventional fiber optic connectors.